A wide area network (WAN) is a computer network that covers a broad geographical area, and generally includes any network whose communications links cross metropolitan, regional, or national boundaries. Typically, WANs cover broader geographical areas than personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs), which are usually limited to a room, building, campus, or specific metropolitan area (e.g. city area), respectively. WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Other WANs are built by Internet service providers and provide connections from an organization's LAN to the Internet. WANs are usually built using leased lines and routers or circuit/packet switching architectures and implement a plurality of network protocols, including Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH).
SONET and SDH are standardized multiplexing protocols that transfer multiple digital bit streams over optical fibers or electrical interfaces. Due to SONET/SDH protocol neutrality and transport-oriented features, SONET/SDH is used for transporting substantially large amounts of telephone calls and data traffic over the same fiber or wire without synchronization problems. SONET/SDH network transmission standards are based on time division multiplexing (TDM). TDM is a technology where two or more signals or bit streams are apparently transferred simultaneously as sub-channels in one communication channel but physically take turns on the channel. This is achieved by dividing the time domain into a plurality of recurrent timeslots, e.g. of about same length, one for each sub-channel. As such, one TDM frame corresponds to one timeslot per sub-channel.